Jayaprakas Yantra
One of the concave bowls of the JayaPrakas instrument is shown above. The entire instrument consists of two such complementary bowls - each a reflection of the sky above - marked in sectors and gap regions. The bowls are complementary, in the sense that, the gap region in one bowl is the sector region in the other and vice versa. The idea being that, the observer needs to be inside the bowl, to take readings - which means that readings would not be possible in the regions where the observer would be able to walk - and hence the complimentary bowl.
How does the instrument work? At the surface level on the depressed bowl - There are pegs in the North-South and East-West direction to hold cross wires. One has to view the shadow of the junction of the cross wires on the bowl of the instrument to determine the co-ordinates of the Sun in the daytime sky. The sectors on the surface of the hemisphere are marked with altitude and azimuth circles, the tropics and intermediate circles (declination parallels) and also circles of the signs of Zodiac. In the Delhi instrument most of these markings are rubbed off. The Altitude and Azimuth circles towards the center of the bowl are still visible, so that Sun measurements in local co-ordinates, close to the Noon time are still relatively easy. For measurements at other times and for Equatorial co-ordinate or elliptical co-ordinate measurements more rigorous calibration will be needed.
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